1. Field of the Invention
The invention relates to viewing aids which are useful for doors and windows, especially for an entrance door of an aeroplane.
2. Description of the Prior Art
U.S. Pat. No. 2,708,136 discloses a prismatic window structure. This is intended for the window adjacent to an aeroplane passenger. Only a single window is provided and the prism is oriented in a horizontal direction to afford the passenger a better forward view. Therefore, the line of sight is essentially parallel to the fuselage. Thus, the proposed structure consists of a single window and the prism affords only a horizontal view for the passenger. The actual prism extends outside of the aeroplane and hence increases the wind resistance.
Various prisms have been proposed in connection with an automobile. An example is U.S. Pat. No. 1,678,479. The prism is disposed near the corner of the automobile windshield to overcome the blind spot created by the corner. To this end, prism strips may be put on the windshield. According to U.S. Pat. No. 1,683,951, an adjustable prism was disposed horizontally near the windshield for observing traffic lights in the car. Similarly, U.S. Pat. No. 1,883,296 discloses a prism which may be tilted in order to better observe traffic lights.
In lieu of a prism, U.S. Pat. No. 3,038,756 suggests the use of two windows in the car, forming an angle with each other.
Furthermore, U.S. Pat. No. 2,312,542 suggests a peep-hole lens combination which permits an observer on one side of a thin door to have a clear view of whatever is present throughout a large range of vision on the other side of the door. However, this lens combination has an insufficient downward angle of view if used in the door of an aeroplane.
U.S. Pat. No. 2,638,810 discloses a door peep observation device wherein a negative lens is provided with a tapered edge and is positioned in a housing with its larger diameter face in abutment with the inner flange portion of the housing. A tube liner of solid kraft paper is adapted to be snugly fitted into the housing and extend from about the inner flange at the front of the housing to the inner edge of the flange of the eye piece holder. By the use of a resilient liner, the leading edge portion thereof may be pressed into the wedge-shaped annular area defined by the tapered edge of the negative lens and the inner wall of the housing. Although this construction provides a tight fit for the negative lens which is relatively tamper proof and water proof, it is not suited for downward observation through a door of an aeroplane.
EP-A 0 829 738 discloses a flat Fresnel mirror for an aeroplane. A large number of ring-like grooves having mutually different diameters are concentrically formed on the back of a planar transparent plastic plate to constitute a Fresnel lens having a concave lens function, a reflecting film is provided to the back so as to provide a convex mirror function to the surface of the transparent plastic plate, and a thin metal sheet is bonded to the reflecting film through an adhesive, thereby constituting a reflecting mirror for an aeroplane cabin. The lamination of the reflecting film and the laminated metal sheet makes the Fresnel lens construction not transparent anymore, therefore it cannot be used as a window component anymore. Also, since the Fresnel lens is concave, it is suited for magnification but not for downward observation through a door of an aeroplane, which requires a convex Fresnel lens.
EP-A 0 020 006 teaches a four-component lens and a display apparatus including a Fresnel lens. The lens structure combines two images that are positioned at a 90 degree angle to each other of which the Fresnel lens is a part. The complicated bulky setup of the structure as described is not suited for a downward observation through a door of an aeroplane.
Finally, EP-A 0 079 895 discloses an aeroplane window comprising a composite window where the inner pane is a prism to provide a good view towards the ground near the aeroplane. The disadvantage of this inner pane with a prism is the bulky design with a weight of about 5 to 6 times compared to the usual thin flat pane with a thickness of approximately 2 mm. Generally, even though it would be preferred, not all doors are supplied with a prism due to the weight restrictions and requirements, which limits the usefulness of the invention. Also, since the prisms and window panes in general are made with non-flame retardant plastic material due to stringent optical requirements, there is an increased danger in case of a fire.
It is frequently desirable to be able to provide a downward angle of view from the window of one or both of the entrance doors of a vehicle such as, for example, those of an aeroplane. This may be desirable for the purpose of observing the outside conditions of the ground close to the aeroplane or the position of the walkway during docking without having to open the doors.
To that end, it has been proposed to enlarge the double panes of the window. However, it has been found that the critical downward angle of view is not thereby appreciably improved.
The present invention relates to viewing aids useful for doors and windows, especially for an entrance door of an aeroplane. These viewing aids comprise, as an essential element, a pane made of a transparent material, which pane has at least one optically active surface comprising a plurality, i.e. at least two, of prisms, or a Fresnel lens, or a combination of both.
In the context of the present invention, an optically active surface is one that diverts incident parallel light beams in such a way that there is not a mere parallel shift of the light beams, but those light beams that have passed the structure with the optically active surface and the incident light beams enclose an angle which is different from 0 or 180 degrees.
An optically active surface is said to comprise a plurality of prisms if it has, in a side view, or in a cross section parallel to one of the longer axes of the pane, the profile of a saw with at least two saw-tooth sections, and, in a top view, or viewed parallel to the short axis of the pane, the lines that connect all neighbouring peak and valley points form a plurality of parallel straight lines.
An optically active surface is said to form a Fresnel lens if a cross section parallel to one of the longer axes of the pane has the profile of a saw, and in a top view, the lines that connect all neighbouring peak and valley points form a series of (optionally concentric) circles, or, less preferred, are elliptical or parabolical.
The angle enclosed between the line connecting all valley points which are separated from each other by exactly one peak point and the lines connecting one specific valley point with the peak points immediately adjacent to the said valley point may be the same for both lines, or it may be preferably different on either side of the valley point (the xe2x80x9cslopexe2x80x9d angle and the xe2x80x9cdraftxe2x80x9d angle). It is possible to keep these angles constant for all prisms formed (all xe2x80x9csaw teethxe2x80x9d have the same profile), which provides a uniform deflection of light beams, or it is possible to gradually increase or decrease either the slope of the draft angle, or both, thereby providing a varying angle of light deflection as the incident beams are shifted along the saw profile.
A viewing aid according to the invention comprises at least one such pane with at least one optically active surface, and preferably also at least one pane which has two substantially flat and substantially parallel planes, or surfaces. It is also possible to use an assembly of three panes, the outer panes both having substantially flat and substantially parallel surfaces, and the centre pane having at least one optically active surface.